In order to reduce greenhouse gas emissions, the geological storage of CO2 is being studied. Enhanced weathering is a carbon dioxide removal technique (CDR technologies) that involves spreading silicate rocks on land and in the oceans to accelerate the natural process of storing carbon as carbonate compounds. Chemical reactions between rocks, water and air remove atmospheric CO2 and store it permanently in solid carbonate minerals. Enhanced weathering uses the process of mineral carbonation, which is the stable, inert and solid fixation of CO2 by the reaction between CO2 and silicate rocks, resulting in the formation of geologically stable carbonate minerals.
What is olivine?
Olivine is a mineral found in volcanic rocks such as basalt, gabbro and peridotite. It is a mineral composed of iron and magnesium silicate, with the chemical formula : (Mg,Fe)2SiO4. The composition of olivine and the content of this mineral vary according to its origin. Olivine is the dominant mineral in peridotites, the rocks that make up the Earth's mantle, with dunite being 90% of olivine. In other magmatic rocks, basalt contains between 10% and 25% olivine and gabbro, a plutonic magmatic rock, contains between 40% and 60% olivine. A technical sheet with the different characteristics of olivine is available here.
What is the potential of olivine to reduce CO2?
Olivine has a strong potential to eliminate CO2 and fight against climate change through enhanced weathering with the alteration of rocks.
This mineral, rich in iron and magnesium silicate, in contact with water induces a chemical reaction that has the capacity to capture and eliminate atmospheric CO2. Its capturing capacity through mineral carbonation releases carbonates and bicarbonates in reaction and thus allows the sequestration of CO2. Studies on the CO2 sequestration capacity of olivine have described a capture potential of over 40,000 Gt of CO2 (L.Turri, 2017).
In the ocean, olivine is a rapidly dissolving mineral. In addition to mitigating rising CO2 concentrations in the atmosphere, this mineral could also mitigate the problem of ocean acidification. The image below shows the spreading of olivine on a beach with the process of enhanced weathering for CO2 sequestration.
Inland, the dissipation of olivine in fields, vacant land or on roads could be another alternative. In the same way as in the ocean, but this time in contact with rainwater, the olivine grains dissolve into carbonate and store CO2.
Enhanced olivine weathering
Enhanced weathering is a way of replicating the natural weathering processes of silicates that play a role in the carbon cycle. Rain dissolves atmospheric CO2, degrades the minerals and releases calcium and magnesium ions. These ions are then transported to the ocean where they precipitate to form carbonates. By precipitating into carbonate, the CO2 is sequestered. Enhanced weathering aims to accelerate the natural dissolution process of minerals by dissipating them on land or on continental coasts and shelves to alkalise the oceans.
For olivine, when water reacts with this rock, the alteration process starts. The CO2 is then mineralised: this is the process of mineral carbonation through the alteration of olivine. At the end of the process, the carbonate is transformed into a limestone rock that has sequestered the CO2.
For more information about this chemical reaction :
Enhanced weathering of olivine has been the subject of various experiments, such as the Vesta project, which aims to dissipate olivine on beaches and wave action breaks it down, thereby accelerating the carbon capture process. This practice needs to take into account the energy costs associated with it. According to Vesta's research, 1 T of olivine could sequester 1.25 T of CO2. The costs of extraction, crushing and transport to land and coast would not be an obstacle for the deployment of this technology. Moreover, Vesta described in its research that in order to capture 100% of annual human emissions, the deployment of enhanced weathering technology could be applied to only 2% of the global continental shelf.
Despite the fact that enhanced weathering involves natural phenomena, there is a need to remain vigilant about the possible environmental impact of enhanced olivine weathering on the soil and for marine biology. Indeed, this practice has not been sufficiently analysed and more research is needed. There are no guarantees about the risks associated with this practice. Olivine is a naturally occurring mineral and permanently present on Earth. Research has described that the mineralisation of this rock is a rapid and feasible process on the human timescale for CO2 reduction. The mineralisation of olivine by enhanced weathering has great potential and must be deployed to limit global warming. This is why companies like Carbon Impact want to accelerate the R&D of the project by developing pilot sites in different geographical areas.
